1. Field of the Invention
The invention relates to apparatus and methods for measuring chromatic dispersion in an optical fiber.
2. Description of the Prior Art
Fiber optics is an important area in the field of modern telecommunications. If an optical signal is to be used over long distances, it is necessary to use "repeaters" to strengthen the signal at intervals. These repeaters are needed because, over long distances, the signal tends to fade, or attenuate, and in the case of digital transmission each optical pulse tends to spread out from the more compact form in which it was transmitted. The pulse spreads because light of different wavelengths travels with different velocities through optical fiber, a phenomenon that is termed dispersion. Since the speeds are different, wavelengths having higher velocity tend to move toward the front of the pulse, while wavelengths having a slower velocity move toward the rear. This causes the entire pulse to spread out, decreasing the clarity of the signal, and causing further problems if the pulse is actually intermingled with other pulses. Thus dispersion can limit the distance over which a signal can be transmitted without regeneration in a repeater. Therefore, it is desirable to be able to find out the dispersion which will occur in optical fiber.
Various methods have been used to calculate dispersion, and attempts have been made to find ways of measuring dispersion which can be used reliably in field applications, as well as the factory. One such method is described in "Measurement of Chromatic Dispersion of Long Spans of Singlemode Fiber: A Factory and Field Test Method," Electronics Letters, Volume 20, No. 4, Feb. 16, 1984, by Vella, Garel-Jones, and Lowe. The method described therein utilizes light having three discrete wavelengths and a reference fiber.
An equation which may be used to describe the time required for a pulse of light of a certain wavelength to traverse an optical fiber of a given length is EQU T=A.lambda..sup.2 +B+C.lambda..sup.-2.
Dispersion is defined as the derivative of the time with respect to the wavelength or D=2A.lambda.-2C.lambda..sup.-3. Neither of these equations describes a straight line, and therefore more than two points would be needed to describe either line, since 2 points are sufficient to specify only a straight line function. For this reason, methods using light of at least three wavelengths have been necessary to find dispersion.